Since an aminoplast obtained by the reaction of a compound having amino group, acid amide bonding or the like, and formaldehyde, as, for example, melamine resin, urea resin, aniline-formaldehyde resin and the like, has an active group such as active hydrogen, active methylol and active alkoxymethylene, it is generally combined with various resins for coating use having a functional group which is reactive with the abovesaid active group, such as hydroxyl groups, isocyanate groups or the like, including acrylic resin, alkyd resin, polyester resin, epoxy resin, polyurethane resin, polyamide resin, polycarbonate resin and the like, and used as heat curing type coating compositions.
Various isocyanate compounds are also used, having combined with said base resins, in heat curing type coating compositions. However, such coating compositions generally require high temperature baking, and when dealing with the combination of said resins and aminoplasts, baking is performed at about 140.degree. C. for the combination of alkyd resin and melamine resin for automotive use and the baking is performed at about 170.degree. C. and more for the combination of epoxy resin and amino resin for coil coating use.
From the standpoint of energy saving, it is, of course, desired to have a lower baking temperature and to this end, various proposals have been made. One of the proposals heretofore made is to increase the acid value and hydroxyl number of resin such as alkyd resins, thereby improving their reactivity with aminoplasts, and to increase the molecular weight of the resin, thereby improving the curing property thereof. However, this inevitably is accompanied by such problems as undesired increase in paint viscosity, lowering of storage stability and decrease in water resistance of the formed film. Attempts have also been made to increase the molecular weight of the aminoplast and improve the curing property of the coating composition but the results have been poor because of the additional problem of poor compatibility with alkyd and other resins.
There is another proposal which involves adding an external catalyst as p-toluene sulfonic acid, phosphoric acid and the like to the combination of coating use resin and crosslinking agent. However, no satisfactory results are obtained because of poor pigment dispersion stability, e.g. segregation, due to the presence of the external acid catalyst, poor storage stability due to the gradual curing of the coating composition at room temperature and the lowering of water resistance of the film.
According to Japanese Patent Application No. 232900/72 filed by the same applicant, the low temperature curing property has been markedly improved by the combination of crosslinking agent and a resin having a resinous acid value based on polycarboxylic acid whose titration midpouint potential in non-aqueous potentiometric titration, in a state capable of developing a resinous acid value, is more than -300 mV, of 2 to 50. However, there is room for improvement in storage stability and even if alcohol, triethylamine or the like is added to strive for some improvement in that respect, there still remains the unsolved question of time color stability of the composition.
From the foregoing, the inventors, having endeavored to obtain a coating composition which comprises a coating resin and crosslinking agent, has an improved curing property without the necessity of using an external catalyst, is curable at a lower temperature or in a shorter period of time, and is excellent in storage stability as well as time color stability, capable of resulting excellent film performance, have completed the invention.